Vet here. *Note I cannot diagnose an animal I have not examined, but I can offer some general education.
If my birds were doing what you described, I would also think respiratory things as well. Potentially Gape worms, but less likely. For respiratory things, there are many, including several viruses and bacteria. The only way to know what it is would be to send in for testing. However, if it is a virus the only thing you can do is supportive care and wait it out. But if it was my birds, what I would do is cover my bases and treat for a bacterial component as well. Most common is Mycoplasma gallisepticum (MG) and Mycoplasma synoviae (MS), especially in backyard flocks. For these, I prefer Tylan (Tylosin) or Oxytetracycline. The tetracyclines also cover a lot of the other bacterial pathogens as well. There are other options out there, but you can generally get a hold of these on your own with the help of google.. and pigeons. You would need a vet to prescribe the others. Also, would separate her from the flock in a climate controlled area as supportive care. Electrolytes in the water and some TLC won't hurt. I really probably wouldn't waste my money on VetRx, and definitely do NOT use colloidal silver..
As others mentioned for the mycoplasms (and technically ILT), birds will remain carriers for life. With the mycoplasms, you may see flare ups periodically and anytime you introduce new birds. It's honestly not that big of a deal. MG is worse, I would probably not sell chicks out of an MG flock. MS isn't ideal, but isn't as bad. Basically, most backyard flocks will get MS at some point and eradicating it in backyard poultry is relatively futile. You are likely to just get it again later. Mortality is not significantly high, although you can still lose some. I've talked to a few poultry vet experts about MS as my breeder became flock MS positive (I was going to quit breeding all poultry because of it), but they basically said don't worry about it. It is a bit of a pain in the butt from time to time, but it's really not worth retiring them all or euthansing a flock for. Some states do have different regulations or requirements on the mycoplasms though, worth checking into for your state. I am currently exploring ways on trying to vaccinate and test myself out of MS over time, I'll write up a thing whenever I figure it out. (Or, at least protect new birds from getting it).
Since you have the tiamulin coming, I would use what you have for a couple weeks to see if she improves. But maybe get a tetracycline on hand for future use if needed. You can also send in testing to a lab yourself. First check with your local state veterinary diagnostic labs, some have some great testing programs for backyard flocks. Otherwise you can send to any lab you want. Just give them a call and they can help guide you on getting samples (and supplies) and what tests they would recommend. University of Minnesota is a big poultry testing lab that lots of poultry vets use as well. (UNL in Nebraska also has a great backyard program for NE flocks). I generally do PCR testing. If she dies, you can also send her in for a necropsy to get a lot of answers.
For any medications used in poultry, I always recommend submitting a FARAD request (usfarad.org). They are wonderful and can advise you on withdrawal times for medications used.
P.S. As you have learned, I definitely recommend quarantining birds for 3-4 weeks, and then the last week I do some "controlled" exposure by swapping a little bit of bedding from each group to get them used to each others pathogens/microbes. If they get sick during this phase, continue quarantine until all symptoms have resolved for a few weeks.